The present invention relates to a device for guiding a blade having a variable pitch angle, such as in particular a blade for directing air admitted into a turbomachine compressor, for example an aviation turbojet or turboprop.
Known guide devices enable the stator blades of a turbomachine to turn about their own axes so as to rectify the flow of gas that has been deflected by the rotor blades. By controlling the extent to which the gas flow is rectified, which is done as a function of the differing speeds of the machine and of the dynamic characteristics of the gas, it is possible to increase performance of a turbomachine.
In most designs, such as those described in documents FR-A-2 743 846, FR-A-2 814 206, and U.S. Pat. No. 5,215,434, such variable-pitch blades are guided in pivoting by bushings of antifriction material, e.g. sintered bronze, fixed in cylindrical bores machined in thickenings of a casing forming the stator of the turbomachine. The pivoting blades are held axially by washers made of a material similar to that of the bushings.
Those guide means generate an opposing torque that requires considerable force to be used to turn the blades, and they present a lifetime that is insufficient. In documents U.S. Pat. No. 3,674,377 and U.S. Pat. No. 2,778,377, proposals are made to replace them by ball bearings, roller bearings, or needle bearings, the axes of rotation of the rollers or the needles being parallel to the pivot axes of the blades. However, and even though the bearings used in those guides do indeed reduce the opposing torque acting against pivoting the blades, the solutions provided do not give entire satisfaction. When aerodynamic forces due to the flow of gas through the turbomachine act on the variable-pitch blades, the rolling elements (balls, needles, or rollers) in the bearings for guiding the blade are not loaded in uniform manner, i.e. some of the rolling elements are subjected to loads that are very high, while other elements are practically not loaded at all. As a consequence, wear on the rolling elements is not uniformly distributed, with highly loaded rolling elements wearing out much more quickly than rolling elements that are not loaded or that are lightly loaded, thereby leading to accelerated aging of the bearings which need to be monitored and changed regularly.